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Intense research is going on to use organic materials in optoelectronic applications like e.g. solar cells. One key element is the diffusion of excitons which depends sensitively on the degree of order in the organic system. Our femtosecond pump-probe experiments on different systems show that it is possible to discriminate between different relaxation scenarios and to provide absolute numbers for local mobilities. In a disordered organic material energetic down hill migration within the inhomogeneous distribution of site energies results in a low exciton mobility. In the case of J-aggregates this phenomenon is suppressed due to the higher degree of order and the excitons are highly mobile throughout their lifetime. Exciton-exciton annihilation results in decay kinetics which depend on the excitation density. The measured decays of the femtosecond absorption signal are modeled for different excitation densities taking exciton diffusion into account. Assuming, that the excitons can move in all three dimensions, results in strong deviations between model and experiment. However, if the mobility of the excitons is restricted to one dimension, almost perfect agreement with the data is achieved. This demonstrates that the aggregates behave like one-dimensional strings and might be considered as photonic wires. |